1. Technical Field
The invention relates to a manufacturing process of a thin film transistor.
2. Related Art
A thin film transistor is widely used as a switching element of a pixel in an electro-optical device such as a liquid crystal device and an organic electro-luminescence (EL) display.
In such thin film transistor, cost reduction is called for in view of a broad range of its use. As a way of meeting such need, it may be conceived to manufacture thin film transistors by semiconductor manufacturing equipment (semiconductor production line) for silicon substrate to provide for sharing the manufacturing equipment, since this contributes to reducing the cost of manufacturing thin film transistors.
By the way, the thin film transistor as the switching element of the electro-optical device is formed on a transparent quartz substrate. However, a substrate detecting sensor detecting a silicon substrate by using light in the semiconductor manufacturing equipment cannot detect the transparent quartz substrate unlike the silicon substrate.
Accordingly, there is a technique of sharing the semiconductor manufacturing equipment by covering a backside of the quartz substrate with a high-melting point metallic film which is impermeable to light, and forming a silicon film thereon, thus making it possible to detect the quartz substrate through the high melting point metallic film which is impermeable to the light of the substrate detecting sensor (for example, refer to JP-3159919).
JP-3159919 is an example of related art.
If the quartz substrate (transparent substrate) is covered in this manner with a high-melting point metallic film, despite covering the high melting point metallic film with a silicon film, for example, due to a high-temperature process at the time of forming a thin film transistor on the quartz substrate, the high-melting point metallic film may diffuse to such an extent that the thin film transistor to be formed may be contaminated.
To prevent metallic contamination of the thin film transistor, it is preferable to use not the high melting point metallic film to cover the quartz substrate but only the silicon film so that the substrate detecting sensor can detect the quartz substrate. At this time, covering the quartz substrate with a silicon film of such a film thickness as to make it more than approximately 5 μm thick enables this silicon film to be impermeable to light like the high melting point metallic film, whereby the quartz substrate can be properly detected by the substrate detection sensor.
But, if the thickness of the silicon film exceeds 5 μm, its film stress grows to cause bowing on the quartz substrate. It becomes impossible to form a proper thin film transistor on a quartz substrate having the bowing. Consequently, it was not practical to share semiconductor manufacturing equipment between the quartz substrate and the silicon substrate and so it was difficult to reduce the cost of the thin film transistor.